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Environmental correlates of condition of the tropical intertidal seagrass, Thalassia hemprichii, in the coastal waters of Singapore
Author
Suryati Mohamed Ali
Supervisor
Goh, Beverly
He, Jie
Abstract
Seagrasses constitute a significant component of coastal marine ecosystems, representing areas that are highly productive and biologically diverse. In Singapore, seagrasses inhabit waters that are turbid and nutrient-rich. This contradicts the assumption that they thrive in oligotrophic waters, and are severely impacted when exposed to excessive nutrient and sediment inputs. With a focus on one tropical seagrass species, Thalassia hemprichii, commonly found on the intertidal shores of Singapore, this study examined seagrass condition across sites with different levels of anthropogenic influence. Assessment of seagrass condition was conducted using physiological (leaf nutrient and photosynthetic pigment content), morphological (leaf size) and community (epiphyte biomass) indicators. Correlates between seagrass condition and environmental variables were elucidated to determine the effectiveness of these bioindicators in assessing environmental quality. The photosynthetic characteristics of T. hemprichii were also studied by measuring their chlorophyll fluorescence and photosynthetic pigment content. In addition, the impacts of elevated water column nutrients on T. hemprichii and its epiphytes were explored in a pilot study that experimentally exposed plants in-situ to two levels of fertilizer addition into the water column. Thalassia hemprichii condition varied significantly across study sites. Better seagrass condition was found at St. John’s Island, situated furthest from anthropogenic activities. Pulau Semakau and Labrador Beach, sites closest to industrial and maritime activities, had poor seagrass condition. Leaf chlorphyll a/b and total chlorophyll/carotenoids ratios were found to be sensitive indicators of light availability and stress conditions, respectively. Elevated levels of water column and sediment nutrients significantly improved Thalassia leaf size and nutrient content. Epiphytic biomass appeared to be an ineffective indicator of water quality as it was not correlated with any environmental variables. Examination of T. hemprichii photosynthetic characteristic found that plants had photoadapted to a low light environment, and had photoprotective mechanisms that were highly responsive to changes in light. Results of the in-situ experimental nutrient addition showed that response of T. hemprichii to higher levels of water column nutrients were variable across treatment plots. Similarly, variation in water column nutrient concentrations at the treatment plot level was high prior and after enrichment. There was some evidence of enrichment on plants exposed to low and high levels of nutrients, observed from reduced leaf C:N and C:P content, lowered leaf chlorophyll a/b ratios, elevated leaf total chlorophyll concentrations, and increased leaf length and area. Increases in water column NOx and PO4 concentrations were also observed at enriched plots. However, results were often non-significant as intra-plot variation was very high. These findings supported the need to improve the current experimental design to overcome the high variation that existed before and after experimental enrichment. Overall, results from this study suggested that the environmental characteristics of seagrass habitats have a great influence on seagrass condition due to the specificity of seagrass response. The findings from this study are highly relevant for the development of indicators of nutrient enrichment, turbidity and photosynthetic stress. This can then be applied to the management and rehabilitation of local and regional seagrass population especially when threatened by eutrophication and excessive sedimentation.
Date Issued
2011
Call Number
QK495.H86 Sur
Date Submitted
2011